Allow upstream to be undici.BalancedPool, pass maxRetriesOn503 to replyOpts

README.md

@@ -124,6 +124,8 @@ be streamed directly to the destination._
 
 An URL (including protocol) that represents the target server to use for proxying.
 
+It can be an array of URLs, if you want to use [`undici.BalancedPool`](https://undici.nodejs.org/#/docs/api/BalancedPool) (`upstream` is compatible with  [`base`](https://github.com/fastify/fastify-reply-from?tab=readme-ov-file#base) option).
+
 ### `prefix`
 
 The prefix to mount this plugin on. All the requests to the current server starting with the given prefix will be proxied to the provided upstream.

index.js

@@ -532,8 +532,10 @@ async function fastifyHttpProxy (fastify, opts) {
 
   const internalRewriteLocationHeader = opts.internalRewriteLocationHeader ?? true
   const oldRewriteHeaders = (opts.replyOptions || {}).rewriteHeaders
+  const maxRetriesOn503 = opts.maxRetriesOn503
   const replyOpts = Object.assign({}, opts.replyOptions, {
-    rewriteHeaders
+    rewriteHeaders,
+    maxRetriesOn503
   })
   fromOpts.rewriteHeaders = rewriteHeaders
 

types/index.d.ts

@@ -60,7 +60,7 @@ declare namespace fastifyHttpProxy {
   ) => string
 
   export interface FastifyHttpProxyOptions extends FastifyReplyFromOptions {
-    upstream: string;
+    upstream: string | string[];
     prefix?: string;
     rewritePrefix?: string;
     proxyPayloads?: boolean;